When operating a tractor-trailer combination, the tractor rear wheels will have substantially more stopping capability than the tractor front wheels since the trailer weight is disposed on the tractor rear wheels. Consequently, tractor air brake systems generally have rear wheel brakes which are considerably larger than the front wheel brakes to take advantage of this difference in stopping capability between the tractor front and rear wheels. The tractor front and rear wheel brakes and the trailer brakes are sized to provide the total brake torque needed when stopping a fully loaded tractor-trailer in a safe distance during a sudden, panic stop. Unfortunately, there is a drawback to using this type of brake system in that the tractor front wheels have a tendency to lock and cause the loss of steering control when stopping a tractor-trailer on wet or icy surfaces.
In order to eliminate this drawback, tractor front wheel brakes have been either disabled or completely removed. This is undesireable because, without tractor front wheel braking, a fully loaded tractor-trailer will not be able to stop in the originally designed safe distance when making a sudden, panic stop due to the inability of the tractor rear wheel brakes and the trailer brakes to produce the designed total brake torque.
There is another condition where tractor front wheel braking is necessary to achieve the best and safest possible stopping. That condition is bobtail operation of a tractor (i.e. without a trailer). When operating a bobtail tractor, the tractor rear wheels will have substantially less stopping capability than the tractor front wheels since there is no trailer weight on the tractor rear wheels. Thus, the tractor rear wheel brakes alone may not safely stop the bobtail tractor. Therefore, tractor front wheel braking is essential.